The present invention relates to a die for producing plastic resin pellets.
The capacity of recently developed extruding apparatus has tended to increase. One type of device uses a die in an underwater cutting system wherein molten resin is extruded into a water medium to form pellets.
This type of device is generally shown in U.S. Pat. No. 3,029,466. That patent shows in FIG. 1 a die having an inlet for resin to be extruded through nozzles 23, and an annular inlet 16 for a heating medium is provided with an outlet 17. Hence, the heating medium circulates only on the inside of the nozzles. Additional heating in the form of helical heating coils 18 are provided on the outer surface of wall 10. Also, in the vicinity of the nozzles steam is supplied via inlet 28 to conduits 24. It is apparent that this die is very complicated, using three discrete types of heating. However, uniformity will still not occur due to different heating rates and the large number of components.
Another type is a hot cutting device wherein molten resin is extruded into a water shower to form pellets. In both cases, the dies employed have been enlarged, with the concomitant increase in the number of nozzles formed in the dies. Since the surface of the die is water-cooled, the temperature of the molten resin through the nozzles is decreased and the viscosity thereof is accordingly increased. As a result, it is difficult to attain smooth flow of the molten resin through the nozzles and the amount of extruded resin is reduced.
Also the enlargement of the dies creates discontinuities in the temperature of the nozzles. This non-uniform temperature results in the creation of pellets of varying sizes. Also the nozzles tend to clog with cooled resin. The forming of the pellets is therefore impossible. In case of a die having a small capacity and a small number of nozzles, it is possible to uniformly heat the nozzle by arranging them on a concentric circle and concentrically providing water jackets inside and outside of the nozzle circle. However, in case of a die having a large capacity and number of nozzles, it is generally not possible to arrange the nozzles on a single concentric circle. Therefore, in this situation, generally, the nozzles are arranged on a plurality of concentric circles. In such arrangement, it is important to uniformly heat and maintain the nozzles at a suitable temperature for extruding.
In view of these requirements, FIGS. 1 and 2 show one example of a prior art extrusion die wherein two grooves 2 and 2' having suitable widths and depths extend from different coaxial positions of one side of a disc-shaped body 1 and two associated annular members 3 and 3' are sealingly inserted into the grooves 2 and 2' to thereby form jackets 4 and 5, respectively. A pair of holes 6 and 7 communicate with the jackets 4 and 5 and are formed in the body 1 in a substantially symmetrical manner, and sleeve-shaped adapters 8 and 9 having a flanged portion are secured to the inlets of the holes 6 and 7. Additionally, a plurality of nozzles 11 each having a small diameter portion 10 in its end are formed in an annular portion between the jackets 4 and 5 in a plurality of parallel lines.
In utilizing the die 12, a molten resin is introduced into the nozzles 11, extruded through the small diameter portions 10 to the outside and cut by a cutter (not shown) into pellets each having a suitable size. A fluidized medium for heating is introduced through the adapter 8 and the hole 6 (FIG. 1) to the jackets 4 and 5 and discharged from the hole 7 and adapter 9. The technique heats the peripheral portions of the nozzles 11.
In such a structure, the nozzles adjacent to the high temperature jackets 4 and 5, such as nozzles 11a and 11b, are heated to a desired temperature, however the nozzle elements spaced the jackets 4 and 5, such as nozzle 11c, has a lower temperature than desired due to heat dissipation and the surrounding temperature. This temperature gradient is generated even between the jacket facing surface and the opposite surface of a single nozzle. In the situation where the device is employed in which molten resin is extruded into the water to abruptly cool the molten metal, the temperature difference due to the position of the nozzles is very large.
FIG. 3 shows a second example of a prior art device. In order to eliminate the above noted defect of temperature gradients, additional jackets 24 and 25 are added between a plurality of the nozzle lines in addition to the jackets 22 and 23. The jackets 24 and 25 are provided along outer and inner circumferences of nozzles 21 in the same manner as in the previous example. Annular members 26 and 27 are inserted into the grooves 24 and 25 in the same manner as the previous example. The other constructional elements are the same as the prior art shown in FIGS. 1 and 2. In the die 28 constructed in accordance with this example, molten resin in the nozzles is uniformly heated because of the addition of the jackets 24 and 25. The local temperature drop as in the first example can therefore be avoided.
However, the second example has another defect that impedes efficiency. Since the jacket is formed with a number of grooves, the minimum distance between the adjacent grooves has a finite limit in view of manufacturing and machining limitations. As a result, the distance between the adjacent grooves tends to be considerable and additionally the distance between the adjacent nozzle elements increases.
Accordingly, the die 28 produced in accordance with this prior art example has an unsatisfactory large size. With the enlargement of the die 28, the associated pellet producing device, that is, the cutter for cutting the extruded resin and the motor for driving the cutter must have a correspondingly large size or capacity. With the enlargement of the cutter diameter, deformations in the cutter blade tend to result so that poor cutting is carried out. Hence although the problem of heating in a uniform manner is overcome, the type of device exhibits other undesirable qualities.
Also, the openings of the jackets 22 to 25 must be clogged by annular members 29, 29', 26 and 27, respectively. This results in a job requiring skill that is time-consuming to clog the jackets with the members 29, 29', 26 and 27. At the same time, the body 20 may be deformed by inserting the annular members into the openings so that the mechanical accuracy in the die 28 will become deteriorated.